Add doc for using UNCOMMENT_TO_RUN

Author: yitchen-tim

TESTING.md

@@ -75,3 +75,16 @@ Simply append the following line to the code cell specified in the Recording sec
 record_utils.playback()
 ```
 Re-running the notebook will use the generated files made during recording.
+
+## Testing commented code
+
+In the example notebooks, some code cells are presented as commented out to prevent accidental execution of costly or long-running operations, particularly those that run on a QPU. However, commented code can become prone to errors over time due to lack of regular testing.
+
+To improve test coverage, we introduce a special flag: `# UNCOMMENT_TO_RUN`. As a developer, you can include this flag within a cell to mark the beginning of a block that should be uncommented during integration testing. The integration test framework will detect the # UNCOMMENT_TO_RUN marker and automatically uncomment all subsequent lines until it encounters a line that is not commented.
+
+To ensure this process works correctly:
+- Use a single `#` for code comments that should be uncommented during testing.
+- Use double `##` for non-code, descriptive comments that should remain commented (and not be treated as executable code).
+
+Following this convention allows the integration test system to selectively and safely test example notebooks without risking unintended costly runs in normal usage.
+